In many situations it is desired to expand a tube radially within a surrounding structure such as a tube sheet, thereby anchoring the tube in the desired position and forming a leak proof joint. One form of swaging used for many years is known as roller swaging. An implement is inserted in the tube and, as it rotates, gradually deforms the tube outwardly. However, roller swaging, while still in common use, is time consuming and is characterized by a tendency to reduce the thickness of the tube wall with accompanying weakening and elongation of the tube.
Preferable swaging techniques are hydraulic. Fluid pressure is applied to the tube internally to produce radial expansion, as described in, for example, U.S. Pat. No. 4,502,308. In other situations, as a preliminary step or where lower pressures are desired, it is preferred to use draw bar swaging in which an elastomeric material is compressed axially, causing it to expand radially within the tube, as described, for example, in U.S. Pat. No. 4,387,507.
By properly swaging a tube, a permanent leak proof joint that will, for example, confine combustion gasses and is not readily subject to corrosion can be formed by eliminating spaces between the tube and the surrounding structure. A positive mechanical interlock can be formed between the tube and the surrounding structure to insure that the tube will not be pulled loose, even if the joint should begin to loosen or leak. The formation of a highly secure mechanical interlock is particularly important in, for example, the boiler of a ship in which many tubes pass through tube sheets that form boiler walls.
The creation of a highly secure mechanical interlock between the tube and the tube sheet may be even more important, in some situations, than the prevention of leakage because a tube that breaks loose from the tube sheet could discharge steam into an area where personnel are present. One common technique for insuring a reliable tube securement is to cause the tube end to be flared or belled at the primary side of the tube sheet to prevent the tube from moving toward the secondary side. This flair provides a positive visually verifiable mechanical interlock and is a standard practice on U.S. Navy ships.
Another common interlock technique is to cause the exposed portion of the tube on the less accessible side of the joint, normally the secondary side of the tube sheet, to bulge or expand permanently to an outside diameter greater than the diameter of the bore in which the tube is located. This bulge prevents the tube from moving toward the primary side of the tube sheet and, like a flared end, provides a positive visually verifiable interlock. This technique is likewise required by the U.S. Navy, which demands a bulge extending 3/8 inches from the secondary side of the tube sheet.
It should be understood that the portion of the tube supported within the tube sheet or other surrounding structure can withstand internal pressure substantially in excess of that required to burst an unsupported tube. It is a common practice to subject this internal portion of the tube to pressures in excess of its burst pressure during formation of a joint by swaging. The exposed and unsupported portions of the tube extending beyond the faces of the tube sheet that are to be bulged and flared are limited to a significantly lower pressure, although the burst pressure of the exposed tube is significantly increased in areas immediately adjacent to the tube sheet when compared to a totally unsupported tube.
Various difficulties have been encountered in connection with the belling and flaring of tubes using both hydraulic fluid and draw bar apparatus. Some of these difficulties are attributable to the substantial increase in diameter that takes place during belling and bulging, thus making it difficult to maintaining an effective seal against the inside of the tube to confine the hydraulic fluid that applies the swaging forces. Another problem encountered in this respect is damage to the seals and seal-backup components.
An objective of the present invention is to provide an improved swaging technique to produce visually verifiable mechanical interlocking by a swaging method that is more efficient and effective when compared to presently known techniques.